There are numerous applications of these sensors: atmospheric pressure measurement, vehicle tire pressure measurement, etc.
Microelectronics technologies make it possible to carry out, on one and the same chip, both the micromachining of mechanical components and the formation of electronic circuits associated with these mechanical components. In a pressure sensor, the mechanical components essentially comprise a cavity sealed off by a deformable diaphragm. The electronic transducer elements comprise strain gages or capacitor plates associated with the diaphragm, and active circuits for detecting the changes in resistance or in capacitance as a function of the deformation of the diaphragm.
The advantage of microelectronics technologies is that they allow the fabrication cost of the sensors to be considerably reduced. Now, the fabrication cost is particularly critical in many applications and especially in commercial applications (for example the automobile market, for detecting tire pressure).
However, the technologies devised hitherto are not optimal from the cost standpoint, in particular because of the complexity introduced by the production of the mechanical components, which requires steps being added to the steps for fabricating the electronic circuit elements.
To give an example, the following solutions have already been proposed:                a (FIG. 1): the joining of two plates, for example a glass plate 10 and a silicon plate 12, bonded together, the empty cavity (denoted by the letter V) being hollowed out right through the thickness of the silicon plate and being sealed off on one side by the glass plate 10 and on the other side by a thin diaphragm 14 that remains in the upper part of the recessed plate 12. Metal strain gages 16, 18 are deposited on this diaphragm, with an insulating layer 20 interposed between the silicon diaphragm 14 and the gages;        b (FIG. 2): similar to FIG. 1, but the strain gages, instead of being produced by deposited metal layers, are doped zones 22, 24 of the silicon of the diaphragm. Their doping is the reverse of that of the diaphragm and said doped zones are isolated from the diaphragm only by the reverse junction formed between these doped zones and the diaphragm. The fabrication is simpler, the piezoresistive sensitivity of these doped zones is very good when the diaphragm deforms, but the isolation is not good and leads to operating defects, notably when the temperature rises;        c (not shown, similar to FIG. 1): the strain gages are portions of a polycrystalline silicon layer deposited on the insulating layer 20, instead of the metal strain gages 16 and 18. The sensitivity is intermediate between that of metal resistance strain gages and that of gages based on doped single-crystal silicon zones,solutions a, b and c requiring a treatment of the silicon plate via its rear face, in order to hollow out the cavity V, thereby appreciably complicating the fabrication, and, additionally, measures having to be taken to protect the strain gages from external, chemical or electrical, attack;        d (FIG. 3): to avoid treating the rear face and to avoid having to protect the strain gages after fabrication, more complex structures comprising two silicon plates have been proposed: a lower plate 12 having the cavity and optionally electronic circuits and strain gages, and an upper plate 26 sealing the cavity and thinned down at the cavity so as to retain only the thin diaphragm 14. This solution is very complex and costly, being reserved for professional applications—notably, it requires onerous measures to be taken to make the electrical output connections from the sensor right through the entire thickness of the first plate 12; and        e (FIG. 4): to eliminate the drawbacks of the above solutions, and to be able to integrate both the mechanical components (cavity, diaphragm) and the electronic circuit elements onto the same substrate, it has been proposed to eliminate the strain gages and to detect the pressure via variation in capacitance, the diaphragm 14 being conductive and constituting one electrode, and another electrode 28 being formed in the silicon substrate. The diaphragm 14 is formed by a polycrystalline silicon layer suspended above the silicon plate 12, the empty cavity V being formed between the plate and the diaphragm. Electronic circuits may be integrated in the silicon plate 12, so as to constitute, on the same substrate, both the mechanical components (cavity, diaphragm) and the integrated measurement circuit. Such a solution is described in U.S. Pat. No. 5,321,989 and in the article by M. Kandler et al. in the Journal of Micromechanics and Microengineering 1992 pp. 199-201 entitled “A miniature single-chip pressure and temperature sensor”. This solution assumes that a large sensor area is consumed, since the capacitances are low, and a wide electrode 28 facing the diaphragm 14 has to be provided. In addition, the pressure measurement is very temperature-dependent and it is necessary in practice to provide a differential measurement with two similar capacitors (to the left and to the right in FIG. 4), one of which is formed with the aid of the diaphragm 14, which can deform under the effect of pressure, and the other is similar but formed from a nondeformable or almost nondeformable diaphragm 141. The relative nondeformability of the second diaphragm may be obtained by depositing thick layers on top of this diaphragm, but the capacitances thus obtained are not sufficiently identical and the temperature compensation is not perfect. This solution has a very large footprint and is therefore expensive. In addition, the diaphragm is conductive and remains subject to external electrical influences that may disturb the measurement. It is also sensitive to chemical influences. Finally, the diaphragm is deposited at 600° C. or higher and must therefore be deposited before certain operations of producing the integrated electronic circuits in the substrate. There is therefore a need to adapt the electronic circuit production according to the steps specific to the mechanical components, and this dependence impedes subsequent circuit production technology developments.        